Time domain measurement of phase noise in a spin torque oscillator

We measure oscillator phase from the zero crossings of the voltage versus time waveform of a spin torque nanocontact oscillating in a vortex mode. The power spectrum of the phase noise varies with Fourier frequency f as 1/f(2), consistent with frequency fluctuations driven by a thermal source. The linewidth implied by phase noise alone is about 70% of that measured using a spectrum analyzer. A phase-locked loop reduces the phase noise for frequencies within its 3 MHz bandwidth.